Semiconductor device

ABSTRACT

An object is to provide a semiconductor device using an oxide semiconductor having stable electric characteristics and high reliability. A transistor including the oxide semiconductor film in which a top surface portion of the oxide semiconductor film is provided with a metal oxide film containing a constituent similar to that of the oxide semiconductor film and functioning as a channel protective film is provided. In addition, the oxide semiconductor film used for an active layer of the transistor is an oxide semiconductor film highly purified to be electrically i-type (intrinsic) by heat treatment in which impurities such as hydrogen, moisture, a hydroxyl group, or a hydride are removed from the oxide semiconductor and oxygen which is a major constituent of the oxide semiconductor and is reduced concurrently with a step of removing impurities is supplied.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 16/011,810, filed Jun. 19, 2018, now allowed, which is a continuation of U.S. application Ser. No. 15/697,504, filed Sep. 7, 2017, now U.S. Pat. No. 10,008,515, which is a continuation of U.S. application Ser. No. 15/231,824, filed Aug. 9, 2016, now U.S. Pat. No. 9,768,199, which is a continuation of U.S. application Ser. No. 14/735,235, filed Jun. 10, 2015, now U.S. Pat. No. 9,431,429, which is a continuation of U.S. application Ser. No. 14/183,755, filed Feb. 19, 2014, now U.S. Pat. No. 9,059,047, which is a continuation of U.S. application Ser. No. 13/080,046, filed Apr. 5, 2011, now U.S. Pat. No. 8,659,013, which claims the benefit of a foreign priority application filed in Japan as Serial No. 2010-090539 on Apr. 9, 2010, all of which are incorporated by reference.

TECHNICAL FIELD

The present invention relates to a semiconductor device and a manufacturing method thereof.

In this specification, a semiconductor device means a general device which can function by utilizing semiconductor characteristics, and an electro-optic device, a semiconductor circuit, and an electronic device are all semiconductor devices.

BACKGROUND ART

A technique by which transistors are formed using semiconductor thin films formed over a substrate having an insulating surface has been attracting attention. Such transistors are applied to a wide range of electronic devices such as an integrated circuit (IC) and an image display device (display device). As semiconductor thin films applicable to the transistors, silicon-based semiconductor materials have been widely used, but oxide semiconductors have been attracting attention as alternative materials.

For example, disclosed is a transistor whose active layer is formed using an amorphous oxide containing indium (In), gallium (Ga), and zinc (Zn) and having an electron carrier concentration of lower than 10¹⁸/cm³ (see Patent Document 1).

A transistor including an oxide semiconductor is known to have a problem of low reliability because of high possibility of change in electric characteristics, although the transistor including an oxide semiconductor can be operated at higher speed than a transistor including amorphous silicon and can be manufactured more easily than a transistor including polycrystalline silicon. For example, a BT test under light is performed, so that the threshold voltage of the transistor fluctuates. On the other hand, Patent Documents 2 and 3 each disclose a technique of preventing charge trapping at the interface of an oxide semiconductor layer with the use of an interfacial stability layer, which is provided on at least one of the top surface and the bottom surface of the oxide semiconductor layer, in order to suppress the shift of the threshold voltage of the transistor including an oxide semiconductor.

REFERENCE

[Patent Document 1] Japanese Published Patent Application No. 2006-165528

[Patent Document 2] Japanese Published Patent Application No. 2010-016347

[Patent Document 3] Japanese Published Patent Application No. 2010-016348

DISCLOSURE OF INVENTION

The transistor disclosed in Patent Document 2 or 3, however, includes as the interfacial stability layer a layer having same characteristics as those of a gate insulating layer and a protective layer, so that the state of the interface with an active layer cannot be kept favorably. This is why it is difficult to suppress charge trapping at the interface between the active layer and the interfacial stability layer. In particular, in the case where the interfacial stability layer and the active layer have equivalent band gaps, charge is likely to be stored.

Thus, a transistor including an oxide semiconductor cannot yet be said to have sufficiently high reliability.

In view of the above problems, an object is to stabilize electric characteristics of a semiconductor device including an oxide semiconductor to increase reliability.

One embodiment of the disclosed invention is based on the following technical idea: a metal oxide film functioning as a channel protective film of the oxide semiconductor film is provided between and in contact with the oxide semiconductor film and contains a constituent similar to that of the oxide semiconductor film. In other words, one embodiment of the disclosed invention includes a layered structure of a metal oxide film and an oxide semiconductor film. Here, containing “a constituent similar to that of the oxide semiconductor film” means containing one or more of metal elements selected from constituents of the oxide semiconductor film.

Such a layered structure makes it possible to sufficiently suppress trapping of charge or the like, which is generated due to the operation of a semiconductor device, or the like, at the interface of the insulating film and the oxide semiconductor film. This advantageous effect is brought by the following mechanism: the metal oxide film containing a material compatible with the oxide semiconductor film is provided in contact with the oxide semiconductor film, whereby suppressed is trapping of charge or the like, which can be generated due to the operation of a semiconductor device, at the interface between the oxide semiconductor film and the metal oxide film.

Since trapping of charge at the interface of the oxide semiconductor film can be suppressed, operation malfunctions of the semiconductor device can be reduced to increase reliability of the semiconductor device.

Further, a structure is preferable in which an insulating film containing a constituent different from the constituents contained in the metal oxide film and the oxide semiconductor film is provided over and in contact with the metal oxide film in such a layered structure. That is, one embodiment of the disclosed invention includes a layered structure in which an oxide semiconductor film, a metal oxide film, and an insulating film are layered.

In such a manner, an insulating film containing a material with which a charge trapping center can be formed at an interface is provided in contact with the metal oxide film, whereby charge can be trapped preferentially at the interface between the metal oxide film and the insulating film compared to the interface between the oxide semiconductor film and the metal oxide film. That is to say, when the insulating film is provided in contact with the metal oxide film, charge is trapped preferentially at the interface of the metal oxide film and the insulating film, so that trapping of charge at the interface between the oxide semiconductor film and the metal oxide film can be suppressed more effectively.

Since trapping of charge at the interface of the oxide semiconductor film can be suppressed and a charge trapping center can be kept away from the oxide semiconductor film, operation malfunctions of the semiconductor device can be reduced to increase reliability of the semiconductor device.

In the above mechanism, the metal oxide film desirably has an enough thickness. This is because the influence of charge trapped at the interface between the metal oxide film and the insulating film may be great when the metal oxide film is thin. For example, the metal oxide film is preferably thicker than the oxide semiconductor film.

The metal oxide film having an insulating property is formed so as not to hinder connection between the oxide semiconductor film and source and drain electrodes, so that resistance can be prevented from being increased as compared to the case where a metal oxide film is provided between an oxide semiconductor film and a source electrode or a drain electrode. Thus, it is possible to suppress deterioration of electric characteristics of the transistor.

When the composition of an oxide semiconductor differs from the stoichiometric composition because of an excess or a deficiency of oxygen, or hydrogen or moisture which serves as an electron donor enters the oxide semiconductor in a thin film formation process, the conductivity of the oxide semiconductor is changed. Such a phenomenon is a factor of a change in electric characteristics of the transistor including such an oxide semiconductor. Therefore, an oxide semiconductor film is highly purified to be electrically i-type (intrinsic) by removing impurities such as hydrogen, moisture, a hydroxyl group, or a hydride (also referred to as a hydrogen compound) from the oxide semiconductor and supplying oxygen which is a major constituent of the oxide semiconductor and is reduced concurrently with a step of removing impurities.

An i-type (intrinsic) oxide semiconductor is an oxide semiconductor highly purified to be i-type (intrinsic) or substantially i-type (intrinsic) by removing hydrogen, which is an n-type impurity, from the oxide semiconductor so that impurities that are not main components of the oxide semiconductor are contained as little as possible.

Note that in the process of making an oxide semiconductor film an i-type oxide semiconductor film, the metal oxide film containing a constituent similar to that of the oxide semiconductor film can also be made an i-type film at the same time. According to one embodiment of the disclosed invention, metal oxide films provided on a top surface and a bottom surface of an oxide semiconductor film are desirably made electrically intrinsic by sufficiently reducing impurities such as moisture and hydrogen.

The electric characteristics of a transistor including a highly-purified oxide semiconductor film, such as the threshold voltage and an off-state current, have almost no temperature dependence. Further, transistor characteristics are less likely to change due to light deterioration.

One embodiment of the present invention will be described in detail below.

One embodiment of the disclosed invention is a semiconductor device including a gate electrode; a gate insulating film covering the gate electrode; an oxide semiconductor film provided over the gate insulating film and in a region overlapping with the gate electrode; a metal oxide film over and in contact with the oxide semiconductor film; and a source electrode and a drain electrode over the metal oxide film and in contact with parts of the oxide semiconductor film. The metal oxide film contains an oxide containing one or more of metal elements selected from constituent elements of the oxide semiconductor film.

Note that, in the above, the semiconductor device may include an insulating film provided over and in contact with the metal oxide film and covering the source electrode and the drain electrode. Further, in the above, a conductive film may be provided over the insulating film.

In the above, the width in a channel length direction of the metal oxide film may be smaller than that of the oxide semiconductor film, and the source electrode and the drain electrode may be in contact with parts of a top surface of the oxide semiconductor film. In the above, side edges in a channel length direction of the oxide semiconductor film may be aligned with those of the metal oxide film. In the above, the metal oxide film may have a structure in which at least the oxide semiconductor film is covered with the metal oxide film, openings are provided so as to expose parts of the oxide semiconductor film, and the source electrode and the drain electrode are in contact with the oxide semiconductor film in the openings. In the above, a second metal oxide film may be provided over and in contact with the gate insulating film and in contact with a bottom surface of the oxide semiconductor film. In the above, the metal oxide film preferably functions as a channel protective film.

In the above, a protective insulating film may be provided over and in contact with the metal oxide film and in contact with parts of top surfaces of the source electrode and the drain electrode. Note that the protective insulating film functions as a film protecting a channel formation region of the oxide semiconductor film when the source electrode and the drain electrode are etched.

In the above, the metal oxide film has preferably a larger energy gap than the oxide semiconductor film. Energy at the bottom of the conduction band of the metal oxide film is preferably higher than that of the oxide semiconductor film.

In the above, the metal oxide film may contain a gallium oxide.

In the above, the gate insulating film may contain a silicon oxide or a hafnium oxide.

In the above, the channel length L of the transistor, which depends on the width in the channel length direction of the metal oxide film functioning as a channel protective film can be greater than or equal to 10 nm and less than or equal to 10 μm, for example, greater than or equal to 0.1 μm and less than or equal to 0.5 μm. The channel length L may be 1 μm or more. Further, the channel width W may be 10 nm or more.

According to one embodiment of the present invention, a transistor having stable electric characteristics can be manufactured.

According to one embodiment of the present invention, a semiconductor device including a highly reliable transistor having favorable electric characteristics can be manufactured.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C are a plan view and cross-sectional views illustrating an embodiment of a semiconductor device;

FIG. 2 is a band diagram of a transistor including an oxide semiconductor film and a metal oxide film;

FIGS. 3A to 3C are a plan view and cross-sectional views illustrating an embodiment of a semiconductor device;

FIGS. 4A to 4H are cross-sectional views each illustrating an embodiment of a semiconductor device;

FIGS. 5A to 5E are cross-sectional views illustrating an example of a manufacturing process of the semiconductor device in FIGS. 1A to 1C;

FIGS. 6A to 6C are views each illustrating an embodiment of a semiconductor device;

FIG. 7 is a cross-sectional view illustrating an embodiment of a semiconductor device;

FIG. 8 is a cross-sectional view illustrating an embodiment of a semiconductor device;

FIG. 9 is a cross-sectional view illustrating an embodiment of a semiconductor device; and

FIGS. 10A to 10F are views illustrating electronic devices.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. Note that the present invention is not limited to the description below, and it is easily understood by those skilled in the art that modes and details disclosed herein can be modified in various ways without departing from the spirit and the scope of the present invention. Thus, the present invention is not construed as being limited to description of the embodiments.

Note that the ordinal numbers such as “first” and “second” in this specification are used for convenience and do not denote the order of steps or the stacking order of layers. In addition, the ordinal numbers in this specification do not denote particular names which specify the present invention.

Embodiment 1

In this embodiment, one embodiment of a semiconductor device and a manufacturing method thereof will be described with reference to FIGS. 1A to 1C, FIG. 2, FIGS. 3A to 3C, FIGS. 4A to 4H, and FIGS. 5A to 5E.

<Structural Example of Semiconductor Device>

In FIGS. 1A to 1C, a cross-sectional view and a plan view of a channel-protective transistor (also referred to as a channel-stop transistor), which is one of bottom-gate transistors, are illustrated as an example of a semiconductor device. FIG. 1A is a plan view and FIGS. 1B and 1C are cross-sectional views taken along line A-B and line C-D in FIG. 1A, respectively. In FIG. 1A, some of components of a transistor 310 (for example, a gate insulating film 402) are omitted for brevity.

The transistor 310 in FIG. 1A includes, over a substrate 400 having an insulating surface, a gate electrode 401, a gate insulating film 402 covering the gate electrode 401, an oxide semiconductor film 403 provided over the gate insulating film 402 and in a region overlapping with the gate electrode 401, a metal oxide film 407 provided over and in contact with the oxide semiconductor film 403, and a source electrode 405 a and a drain electrode 405 b provided over the metal oxide film 407 and in contact with parts of the oxide semiconductor film 403. The metal oxide film 407 functions as a channel protective film in the transistor 310 in FIG. 1A. Further, the transistor 310 preferably includes an insulating film 409 covering the source electrode 405 a and the drain electrode 405 b and over and in contact with the metal oxide film 407.

Here, it is desirable to use an oxide containing a constituent similar to that of the oxide semiconductor film 403 for the metal oxide film 407. Specifically, the metal oxide film 407 is preferably a film containing an oxide containing one or more of metal elements selected from constituent elements of the oxide semiconductor film. This is because such a material is compatible with the oxide semiconductor film 403 and thus, when it is used for the metal oxide film 407, the state of the interface with the oxide semiconductor film can be kept favorably. That is to say, the use of the above material for the metal oxide film 407 makes it possible to suppress trapping of charge at the interface between the oxide semiconductor film and the metal oxide film in contact with the oxide semiconductor film (here, the interface between the metal oxide film 407 and the oxide semiconductor film 403).

The metal oxide film 407 needs to have a larger energy gap than the oxide semiconductor film 403 because the oxide semiconductor film 403 is used as an active layer. In addition, it is necessary to form at least an energy barrier between the metal oxide film 407 and the oxide semiconductor film 403, in which carriers do not flow from the oxide semiconductor film 403 at room temperature (20° C.). For example, the energy difference between the bottom of the conduction band of the metal oxide film 407 and the bottom of the conduction band of the oxide semiconductor film 403 or the energy difference between the top of the valence band of the oxide semiconductor film 403 and the top of the valence band of the metal oxide film 407 is desirably 0.5 eV or more, more desirably 0.7 eV or more. In addition, the energy difference therebetween is desirably 1.5 eV or less.

Specifically, for example, when an In—Ga—Zn—O-based material is used for the oxide semiconductor film 403, the metal oxide film 407 may be formed using a material containing gallium oxide, or the like. When the gallium oxide is in contact with the In—Ga—Zn—O-based material, the energy barrier is about 0.8 eV on the conduction band side and about 0.9 eV on the valence band side.

Note that a gallium oxide is also referred to as GaO_(x) and the value of x is preferably set so that the oxygen amount exceeds the stoichiometric proportion. For example, the value of x is preferably set to larger than or equal to 1.4 and smaller than or equal to 2.0, further preferably larger than or equal to 1.5 and smaller than or equal to 1.8. When a gallium oxide is used as the metal oxide film 407, it is desirable that a gallium oxide film is a film from which impurities such as hydrogen and water are sufficiently reduced. Note that a gallium oxide film may contain an impurity element other than hydrogen, e.g., an element belonging to Group 3 such as yttrium, an element belonging to Group 4 such as hafnium, an element belonging to Group 13 such as aluminum, an element belonging to Group 14 such as silicon, or nitrogen so that the energy gap of the gallium oxide is increased to improve the insulating property. The energy gap of a gallium oxide film which does not contain an impurity is 4.9 eV; however, when the gallium oxide film contains any of the above impurities at about greater than 0 atomic % and less than or equal to 20 atomic %, the energy gap can be increased to about 6 eV.

In order to reduce charge sources and charge trapping centers, it is desirable to sufficiently reduce impurities such as hydrogen and water in the metal oxide film. This idea is similar to the idea of reduction of impurities in an oxide semiconductor film.

The metal oxide film 407 functioning as a channel protective film is provided in a region overlapping with a channel formation region of the oxide semiconductor film 403; thus, damage to the channel formation region by etching of the source electrode 405 a and the drain electrode 405 b (e.g., damage due to plasma or etchant in etching) can be prevented. Thus, a semiconductor device including an oxide semiconductor with stable electric characteristics can be provided.

The source electrode 405 a and the drain electrode 405 b are in contact with parts of a top surface of the oxide semiconductor film 403, when the width in the channel length direction of the metal oxide film 407 is smaller than that of the oxide semiconductor film 403 as in FIG. 1B. That is, the width in the channel length direction of the metal oxide film 407 is made small, so that the channel length of the transistor 310 is made small; therefore, an increase in operation speed and a reduction in power consumption of the transistor can be achieved.

The transistor 310 has a structure in which a contact area between the source and drain electrodes 405 a and 405 b and the oxide semiconductor film 403 is reduced compared to a so-called channel-etched transistor in which the metal oxide film 407 is not provided; thus, a region around the interface between the source and drain electrodes 405 a and 405 b and the oxide semiconductor film 403 becomes a highly resistant region. Thus, the concentration of the electric field in the transistor 310 can be alleviated, whereby a short-channel effect can be suppressed even when the transistor 310 is downsized.

When the insulating film 409 is provided over and in contact with the metal oxide film 407, it is desirable to use a material with which a charge trapping center can be formed at the interface with the metal oxide film 407 when the material is in contact with the metal oxide film 407, for the insulating film 409. By using such a material for the insulating film 409, charge is preferentially trapped at the interface between the insulating film 409 and the metal oxide film 407, so that it is possible to effectively suppress trapping of charge at the interface between the metal oxide film 407 and the oxide semiconductor film 403. Note that when many charge trapping centers are formed at the interface between the insulating film 409 and the metal oxide film 407, transistor characteristics might possibly get worse; thus, it is favorable that charge trapping centers be slightly more likely to be formed at the interface between the insulating film 409 and the metal oxide film 407 than at the interface between the oxide semiconductor film 403 and the metal oxide film 407.

Specifically, the insulating film 409 may be formed to have a single-layer or layered structure using any of a silicon oxide, a silicon nitride, an aluminum oxide, an aluminum nitride, a mixed material of any of them, and the like. For example, when a material containing a gallium oxide is used for the metal oxide film 407, a silicon oxide, a silicon nitride, or the like is preferably used for the insulating film 409. In addition, the energy gap of the insulating film 409 is desirably larger than that of the metal oxide film 407 because the insulating film 409 is in contact with the metal oxide film 407.

Note that it is not necessary to limit the material of the insulating film 409 to the above as long as a charge trapping center can be formed at the interface between the insulating film 409 and the metal oxide film 407. Further, treatment through which a charge trapping center is formed may be performed on the interface between the insulating film 409 and the metal oxide film 407. As such treatment, plasma treatment and treatment for adding an element (ion implantation or the like) are given, for example.

Note that the metal oxide film 407 is patterned into an island shape in the transistor 310; however, the metal oxide film 407 is not necessarily patterned into an island shape. In addition, side edges in a channel length direction of the oxide semiconductor film 403 may be aligned with those of the metal oxide film 407. When the insulating film 409 is formed, a second gate electrode may be further formed over the oxide semiconductor film 403. In this case, the transistor 310 may be a top-gate transistor in which the gate electrode 401 is not provided. A second metal oxide film may be further provided over and in contact with the gate insulating film 402. The oxide semiconductor film 403 may be patterned so that the width in the channel length direction of the oxide semiconductor film 403 is smaller than that of the gate electrode 401. An insulating film may be further provided over the transistor 310. Further, openings may be formed in the gate insulating film 402, the metal oxide film 407, the insulating film 409, and the like in order that the source electrode 405 a and the drain electrode 405 b may be electrically connected to a wiring. Note that it is not always necessary but desirable to process the oxide semiconductor film 403 into an island shape.

FIG. 2 is an energy band diagram (schematic diagram) of the transistor 310, that is, an energy band diagram of the structure where the insulating film, the oxide semiconductor film, the metal oxide film, and the insulating film are bonded to each other from the gate electrode GE side, and E_(F) denotes the Fermi level of the oxide semiconductor film. FIG. 2 shows the case where a silicon oxide (SiO_(x)) (with a band gap Eg of 8 eV to 9 eV), a gallium oxide (GaO_(x)) (with a band gap Eg of 4.9 eV), and an In—Ga—Zn—O-based non-single-crystal film (with a band gap Eg of 3.15 eV) are used as the insulating film, the metal oxide film, and the oxide semiconductor (OS) film, respectively, on the assumption of the ideal state where the insulating films, the metal oxide films, and the oxide semiconductor film are all intrinsic. Note that the energy difference between the vacuum level and the bottom of the conduction band of the silicon oxide is 0.95 eV, the energy difference between the vacuum level and the bottom of the conduction band of the gallium oxide is 3.5 eV, and the energy difference between the vacuum level and the bottom of the conduction band of the In—Ga—Zn—O-based non-single-crystal film is 4.3 eV.

As shown in FIG. 2, on the gate electrode side (the channel side) of the oxide semiconductor film, energy barriers of about 3.35 eV and about 2.5 eV exist at the interface between the oxide semiconductor film and the insulating film. On the side opposite to the gate electrode (the back channel side) of the oxide semiconductor film, similarly, energy barriers of about 0.8 eV and about 0.95 eV exist at the interface between the oxide semiconductor film and the metal oxide film. When such energy barriers exist at the interface between the oxide semiconductor film and the insulating film and at the interface between the oxide semiconductor film and the metal oxide film, transport of carriers at the interfaces can be prevented; thus, the carriers do not travel from the oxide semiconductor film to the insulating film and from the oxide semiconductor film to the metal oxide film, and the carriers travel through the oxide semiconductor film. In other words, the oxide semiconductor film is provided so as to be sandwiched between materials whose band gaps are each larger than that of the oxide semiconductor film (here, the metal oxide film and the insulating film), whereby the carriers travel through the oxide semiconductor film.

FIGS. 3A to 3C and FIGS. 4A to 4H illustrate structural examples of transistors having different structures from that in FIGS. 1A to 1C.

FIGS. 3A to 3C illustrate cross-sectional views and a plan view of a transistor having a structure in which the metal oxide film 407 covers the oxide semiconductor film 403. Here, FIG. 3A is a plan view, FIGS. 3B and 3C are cross-sectional views taken along line A-B and line C-D in FIG. 3A, respectively. Note that in FIG. 3A, some of components of a transistor 320 (e.g., the gate insulating film 402) are omitted for brevity.

The transistor 320 in FIG. 3A is the same as the transistor 310 in FIGS. 1A to 1C in that it includes, over the substrate 400, the gate electrode 401, the gate insulating film 402, the oxide semiconductor film 403, the metal oxide film 407, the source electrode 405 a, the drain electrode 405 b, and the insulating film 409. The difference between the transistor 320 in FIG. 3A and the transistor 310 in FIGS. 1A to 1C is that the metal oxide film 407 covers the oxide semiconductor film 403. Here, in the transistor 320, the source and drain electrodes 405 a and 405 b are in contact with the oxide semiconductor film 403 in openings formed in the metal oxide film 407 so as to expose parts of the oxide semiconductor film 403. The other components are the same as those of the transistor 310 in FIGS. 1A to 1C; thus, the description on FIGS. 1A to 1C can be referred to for the details.

With such a structure, the transistor 320 has a structure in which a contact area between the source and drain electrode 405 a and 405 b and the oxide semiconductor film 403 is reduced compared to the transistor 310 in FIGS. 1A to 1C; thus, a region around the interface between the source and drain electrodes 405 a and 405 b and the oxide semiconductor film 403 becomes a more highly resistant region. Thus, the concentration of the electric field in the transistor 310 can be further alleviated, whereby a short-channel effect can be suppressed more effectively even when the transistor 310 is downsized.

Transistors 330 and 340 in FIGS. 4A and 4B are different from the transistors 310 and 320, respectively, in that a conductive film 410 is provided over the insulating film 409 and in a region overlapping with a channel formation region of the oxide semiconductor film 403. The conductive film 410 may be formed using a material and a method which are similar to those of the gate electrode 401. The other components are the same as those of the transistors 310 and 320. Note that transistors 350 and 360 in FIGS. 4C and 4D are different from the transistors 330 and 340 in that the gate electrode 401 and the gate insulating film 402 are not provided and the transistors 350 and 360 are top-gate transistors.

Transistors 370 and 380 in FIGS. 4E and 4F are different from the transistors 310 and 320 in that the metal oxide film 404 is further provided over and in contact with the gate insulating film 402. The metal oxide film 404 may be formed using a material and a method which are similar to those of the metal oxide film 407. The gate insulating film 402 is preferably formed using a material and a method which are similar to those of the insulating film 409. The other components are the same as those of the transistors 310 and 320.

With such a structure, trapping of charge can be suppressed even at a bottom surface portion of the oxide semiconductor film 403, that is, at the interface between the oxide semiconductor film 403 and the metal oxide film 404. By using such a material with which a charge trapping center can be formed at the interface with the metal oxide film 404 when the material is in contact with the metal oxide film 404, for the gate insulating film 402, charge is trapped preferentially at the interface between the gate insulating film 402 and the metal oxide film 404, so that trapping of charge at the interface between the metal oxide film 404 and the oxide semiconductor film 403 can be suppressed more effectively.

The transistor 390 in FIG. 4G is different from the transistor 310 in that the oxide semiconductor film 403 is patterned so that the width in the channel length direction of the oxide semiconductor film 403 is smaller than that of the gate electrode 401. The other components are the same as those of the transistor 310. With such a structure, the oxide semiconductor film 403 can have a flat shape, so that carrier scattering can be prevented and interface levels can be reduced at the interface between the oxide semiconductor film 403 and the gate insulating film 402.

A transistor 500 in FIG. 4H is different from the transistor 310 in that a protective insulating film 419 is further provided over and in contact with the metal oxide film 407. That is, parts of a top surface of the protective insulating film 419 are in contact with the source electrode 405 a and the drain electrode 405 b, and the protective insulating film 419 functions as a channel protective film together with the metal oxide film 407. The protective insulating film 419 may be formed using a material and a method which are similar to those of the insulating film 409. The other components are the same as those of the transistors 310.

With such a structure, even if the insulating film 409 is not provided, the protective insulating film 419 can be provided over and in contact with the metal oxide film 407 using such a material with which a charge trapping center can be formed at the interface with the metal oxide film 407 when the material is in contact with the metal oxide film 407. Thus, even if the insulating film 409 is not provided, charge is trapped preferentially at the interface between the protective insulating film 419 and the metal oxide film 407 compared to at the interface between the oxide semiconductor film 403 and the metal oxide film 407, so that trapping of charge at the interface between the metal oxide film 407 and the oxide semiconductor film 403 can be suppressed more effectively.

Note that the structures of the transistors can be combined with each other as appropriate.

<Example of Manufacturing Process of Transistor>

Hereinafter, examples of manufacturing processes of the transistors illustrated in FIGS. 1A to 1C, FIGS. 3A to 3C, and FIGS. 4A to 4H will be described with reference to FIGS. 5A to 5E.

<Manufacturing Process of Transistor 310>

An example of a manufacturing process of the transistor 310 illustrated in FIGS. 1A to 1C will be described with reference to FIGS. 5A to 5E.

First, a conductive film is formed over the substrate 400 having an insulating surface, and then, the gate electrode 401 is formed in a first photolithography step (see FIG. 5A). Note that a resist mask may be formed by an inkjet method. Formation of the resist mask by an inkjet method needs no photomask; thus, manufacturing cost can be reduced.

Although there is no particular limitation on a substrate which can be used as the substrate 400 having an insulating surface, it is necessary that the substrate have at least enough heat resistance to heat treatment to be performed later. For example, a glass substrate, a ceramic substrate, a quartz substrate, or a sapphire substrate can be used. Alternatively, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate of silicon, silicon carbide, or the like; a compound semiconductor substrate of silicon germanium or the like; an SOI substrate, or the like can be used as long as the substrate has an insulating surface. In addition, semiconductor elements may be provided over these substrates.

A flexible substrate may be used as the substrate 400. When a flexible substrate is used, a transistor including the oxide semiconductor film 403 may be directly formed over the flexible substrate. Alternatively, a transistor including the oxide semiconductor film 403 may be formed over a manufacturing substrate, and then, the transistor may be separated and transferred to a flexible substrate. Note that in order to separate and transfer a transistor including the oxide semiconductor film 403 from the manufacturing substrate to the flexible substrate, a separation layer is preferably provided between the manufacturing substrate and the transistor including the oxide semiconductor film 403.

An insulating film functioning as a base film may be provided between the substrate 400 and the gate electrode 401. The base film may have a function of preventing diffusion of an impurity element from the substrate 400, and can be formed with a single-layer or layered structure using one or more of a silicon nitride film, a silicon oxide film, a silicon nitride oxide film, and a silicon oxynitride film.

The gate electrode 401 can be formed with a single layer or a stack layer using a metal material such as molybdenum, titanium, tantalum, tungsten, aluminum, copper, neodymium, or scandium, or an alloy material which includes any of these materials as a main component.

Next, the gate insulating film 402 is formed over the gate electrode 401 (see FIG. 5A).

Specifically, the gate insulating film 402 may be formed to have a single-layer or layered structure using any of a silicon oxide film, a silicon nitride film, a silicon oxynitride film, a silicon nitride oxide film, an aluminum oxide film, an aluminum nitride film, an aluminum oxynitride film, an aluminum nitride oxide film, a hafnium oxide film, and a gallium oxide film.

There is no particular limitation on the method for forming the gate insulating film 402, and for example, the gate insulating film 402 may be formed by a deposition method such as a plasma CVD method or a sputtering method.

Note that after the gate insulating film 402 is formed, the metal oxide film 404 is further formed over the gate insulating film 402, so that the transistor 370 in FIG. 4E and the transistor 380 in FIG. 4F can be formed. The metal oxide film 404 can be formed using a material and a method which are similar to those of the metal oxide film 407 to be described later.

Next, the oxide semiconductor film 403 having a thickness of greater than or equal to 3 nm and less than or equal to 30 nm is formed over the gate insulating film 402 by a sputtering method (see FIG. 5A). The above thickness is preferable because the transistor might possibly be normally on when the oxide semiconductor film 403 is too thick (e.g., the thickness is 50 nm or more). Note that the gate insulating film 402 and the oxide semiconductor film 403 is preferably formed successively without being exposed to the air.

Note that before the oxide semiconductor film 403 is formed by a sputtering method, powdery substances (also referred to as particles or dust) which are attached on a surface of the gate insulating film 402 are preferably removed by reverse sputtering in which an argon gas is introduced and plasma is generated. The reverse sputtering refers to a method in which a voltage is applied to a substrate side to generate plasma in the vicinity of the substrate to modify a surface. Note that instead of argon, a gas of nitrogen, helium, oxygen or the like may be used.

As an oxide semiconductor used for the oxide semiconductor film 403, any of the following oxide semiconductors can be used: an In—Sn—Ga—Zn—O-based oxide semiconductor which is an oxide of four metal elements; an In—Ga—Zn—O-based oxide semiconductor, an In—Sn—Zn—O-based oxide semiconductor, an In—Al—Zn—O-based oxide semiconductor, a Sn—Ga—Zn—O-based oxide semiconductor, an Al—Ga—Zn—O-based oxide semiconductor, and a Sn—Al—Zn—O-based oxide semiconductor which are oxides of three metal elements; an In—Zn—O-based oxide semiconductor, a Sn—Zn—O-based oxide semiconductor, an Al—Zn—O-based oxide semiconductor, a Zn—Mg—O-based oxide semiconductor, a Sn—Mg—O-based oxide semiconductor, and an In—Mg—O-based oxide semiconductor which are oxides of two metal elements; and an In—O-based oxide semiconductor, a Sn—O-based oxide semiconductor, and a Zn—O-based oxide semiconductor which are oxides of one metal element. Further, SiO₂ may be contained in the above oxide semiconductor. In this specification, for example, an In—Ga—Zn—O-based oxide semiconductor means an oxide film containing indium (In), gallium (Ga), and zinc (Zn), and there is no particular limitation on the composition ratio. The In—Ga—Zn—O-based oxide semiconductor may contain an element other than In, Ga, and Zn.

As the oxide semiconductor film 403, a thin film formed using a material expressed by a chemical formula of InMO₃(ZnO)_(m) (m>0) can be used. Here, M represents one or more metal elements selected from Ga, Al, Mn, and Co. For example, M can be Ga, Ga and Al, Ga and Mn, Ga and Co, or the like.

In this embodiment, the oxide semiconductor film 403 is formed by a sputtering method using an In—Ga—Zn—O-based oxide semiconductor target for film formation. Alternatively, the oxide semiconductor film 403 can be formed by a sputtering method under a rare gas (typically, argon) atmosphere, an oxygen atmosphere, or a mixed atmosphere of a rare gas and oxygen.

As a target for forming an In—Ga—Zn—O-based oxide semiconductor film as the oxide semiconductor film 403 by a sputtering method, for example, an oxide semiconductor film formation target with the following composition ratio may be used: the composition ratio of In₂O₃:Ga₂O₃:ZnO is 1:1:1 [molar ratio]. Note that it is not necessary to limit the material and the composition ratio of the target to the above. For example, an oxide semiconductor film formation target with the following composition ratio may alternatively be used: the composition ratio of In₂O₃:Ga₂O₃:ZnO is 1:1:2 [molar ratio].

When an In—Zn—O-based material is used for the oxide semiconductor, a target with the following composition ratio is used: the composition ratio of In:Zn is 50:1 to 1:2 in an atomic ratio (In₂O₃:ZnO=25:1 to 1:4 in a molar ratio), preferably 20:1 to 1:1 in an atomic ratio (In₂O₃:ZnO=10:1 to 1:2 in a molar ratio), further preferably 15:1 to 1.5:1 in an atomic ratio (In₂O₃:ZnO=15:2 to 3:4 in a molar ratio). For example, a target used for the formation of an In—Zn—O-based oxide semiconductor has the following atomic ratio: the atomic ratio of In:Zn:O is X:Y:Z, where Z>1.5X+Y.

In addition, the filling factor of the oxide semiconductor target for film formation is greater than or equal to 90% and less than or equal to 100%, preferably greater than or equal to 95% and less than or equal to 99.9%. With the use of the oxide semiconductor target for film formation with a high filling factor, the oxide semiconductor film 403 can be formed to be dense.

A high-purity gas in which impurities such as hydrogen, water, hydroxyl, and hydride are removed is preferably used as a sputtering gas used in formation of the oxide semiconductor film 403.

For the formation of the oxide semiconductor film 403, a substrate is held in a deposition chamber kept at reduced pressure and the substrate temperature is preferably set at higher than or equal to 100° C. and lower than or equal to 600° C., preferably higher than or equal to 200° C. and lower than or equal to 400° C. The film formation is performed while the substrate 400 is heated, the concentration of impurities contained in the oxide semiconductor film 403 can be reduced. Moreover, damage to the oxide semiconductor film 403 due to sputtering is reduced. Then, a sputtering gas from which hydrogen and moisture are removed is introduced into the deposition chamber from which residual moisture is being removed, and the oxide semiconductor film 403 is formed over the substrate 400 with the use of the target. In order to remove residual moisture in the deposition chamber, an entrapment vacuum pump such as a cryopump, an ion pump, or a titanium sublimation pump is preferably used. As an evacuation unit, a turbo pump to which a cold trap is added may be used. In the deposition chamber which is evacuated with the cryopump, for example, a hydrogen atom, a compound containing a hydrogen atom, such as water (H₂O), (further preferably, also a compound containing a carbon atom), and the like are removed, whereby the concentration of impurities in the oxide semiconductor film 403 formed in the deposition chamber can be reduced.

One example of the deposition condition is as follows: the distance between the substrate and the target is 100 mm, the pressure is 0.6 Pa, the direct-current (DC) power source is 0.5 kW, and the atmosphere is an oxygen atmosphere (the rate of the oxygen flow is 100%). Note that a pulsed direct current power source is preferably used because powdery substances (also referred to as particles or dust) generated in film formation can be reduced and thickness distribution can be small.

After that, heat treatment (first heat treatment) is preferably performed on the oxide semiconductor film 403. Excessive hydrogen (including water and a hydroxyl group) in the oxide semiconductor film 403 is removed by the first heat treatment and a structure of the oxide semiconductor film 403 is improved, so that defect levels in energy gap of the oxide semiconductor film 403 can be reduced. The first heat treatment is performed at a temperature higher than or equal to 250° C. and lower than or equal to 700° C., preferably higher than or equal to 450° C. and lower than or equal to 600° C. The temperature of the first heat treatment is preferably lower than the strain point of the substrate.

The heat treatment can be performed in such a way that, for example, an object to be heated is introduced into an electric furnace in which a resistance heating element or the like is used and heated at 450° C. in a nitrogen atmosphere for an hour. During the heat treatment, the oxide semiconductor film 403 is not exposed to the air, in order to prevent entry of water and hydrogen.

The heat treatment apparatus is not limited to an electric furnace; the heat treatment apparatus can be an apparatus that heats an object to be heated using thermal radiation or thermal conduction from a medium such as a heated gas. For example, an RTA (rapid thermal annealing) apparatus such as a GRTA (gas rapid thermal annealing) apparatus or an LRTA (lamp rapid thermal annealing) apparatus can be used. An LRTA apparatus is an apparatus for heating an object using radiation of light (an electromagnetic wave) emitted from a lamp such as a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp, a high-pressure sodium lamp, or a high-pressure mercury lamp. A GRTA apparatus is an apparatus for heat treatment using a high-temperature gas. As the gas, an inert gas which does not react with an object in heat treatment, such as nitrogen or a rare gas like argon, is used.

For example, as the first heat treatment, GRTA treatment may be performed in the following manner. The object is put in an inert gas atmosphere that has been heated, heated for several minutes, and then taken out of the inert gas atmosphere. The GRTA treatment enables high-temperature heat treatment in a short time. Moreover, in the GRTA treatment, even conditions of the temperature that exceeds the upper temperature limit of the object can be employed. Note that the gas may be switched from the inert gas to a gas including oxygen during the process. This is because defect levels in the energy gap due to oxygen deficiency can be reduced by performing the first heat treatment in an atmosphere including oxygen.

Note that as the inert gas atmosphere, an atmosphere that contains nitrogen or a rare gas (e.g., helium, neon, or argon) as its main component and does not contain water, hydrogen, and the like is preferably used. For example, the purity of nitrogen or a rare gas such as helium, neon, or argon introduced into a heat treatment apparatus is set to higher than or equal to 6N (99.9999%), preferably higher than or equal to 7N (99.99999%) (i.e., the impurity concentration is lower than or equal to 1 ppm, preferably lower than or equal to 0.1 ppm).

In any case, when impurities are reduced by the first heat treatment to form the oxide semiconductor film 403 that is an i-type (intrinsic) or substantially i-type semiconductor, a transistor with extremely excellent characteristics can be realized.

The above heat treatment (first heat treatment) has an effect of removing hydrogen, water, and the like and thus can be referred to as dehydration treatment, dehydrogenation treatment, or the like. The dehydration treatment or the dehydrogenation treatment can be performed, for example, after the oxide semiconductor film 403 is processed into an island shape. Such dehydration treatment or dehydrogenation treatment may be conducted once or plural times.

Next, the oxide semiconductor film 403 is preferably processed to have an island-shaped oxide semiconductor film 403 in a second photolithography step (see FIG. 5A). A resist mask for forming the island-shaped oxide semiconductor film 403 may be formed by an inkjet method. Formation of the resist mask by an inkjet method needs no photomask; thus, manufacturing cost can be reduced. For the etching of the oxide semiconductor film 403, wet etching, dry etching, or both of them may be employed.

Note that the island-shaped oxide semiconductor film 403 is processed so that the width in the channel length direction of the oxide semiconductor film 403 is less than that of the gate electrode 401, so that the transistor 390 in FIG. 4G can be formed.

Next, plasma treatment may be performed using a gas such as N₂O, N₂, or Ar so that water adsorbed to a surface of an exposed portion of the oxide semiconductor film 403 is removed. When plasma treatment is performed, the metal oxide film 407 is preferably formed in contact with the oxide semiconductor film 403 without exposure to the air, following the plasma treatment.

Next, the metal oxide film 427 is formed to cover the oxide semiconductor film 403 (see FIG. 5B). Note that the metal oxide film 427 is processed into an island shape in a later step to be the metal oxide film 407.

Here, the metal oxide film 427 (the metal oxide film 407) desirably contains a constituent similar to that of the oxide semiconductor film 403 and is desirably formed using an oxide containing the main constituent material of the oxide semiconductor film 403. Such a material is compatible with the oxide semiconductor film 403; thus, when it is used for the metal oxide film 407, the state of the interface between the oxide semiconductor film and the metal oxide film 407 can be kept favorably. That is to say, the use of the above material for the metal oxide film 427 (the metal oxide film 407) makes it possible to suppress trapping of charge at the interface between the metal oxide film 407 and the oxide semiconductor film 403.

The metal oxide film 407 needs to have a larger energy gap than the oxide semiconductor film 403. In addition, it is necessary to form at least an energy barrier between the metal oxide film 407 and the oxide semiconductor film 403, in which carriers do not flow from the oxide semiconductor film 403 at room temperature (20° C.).

In order to reduce charge sources and charge trapping centers, it is desirable to sufficiently reduce impurities such as hydrogen and water in the metal oxide film 407. This idea is similar to the idea of reduction of impurities in an oxide semiconductor film.

The metal oxide film 427 (the metal oxide film 407) is preferably formed by a method by which impurities such as water and hydrogen do not enter the metal oxide film 427. When hydrogen is contained in the metal oxide film 427 (the metal oxide film 407), entry of the hydrogen into the oxide semiconductor film 403 or extraction of oxygen in the oxide semiconductor film 403 by hydrogen may occur, thereby causing the backchannel of the oxide semiconductor film 403 to have lower resistance (to be n-type), so that a parasitic channel may be formed. Thus, it is important that a deposition method in which hydrogen is not used is employed such that the metal oxide film 427 (the metal oxide film 407) contains hydrogen as little as possible.

Therefore, the metal oxide film 427 is preferably formed by a sputtering method, and a high-purity gas in which impurities such as hydrogen, water, a hydroxyl group, and hydride are removed is preferably used as a sputtering gas used for film formation.

The metal oxide film 427 (the metal oxide film 407) preferably has a thickness large enough to keep a charge trapping center away from the oxide semiconductor film 403. Specifically, the metal oxide film 427 (the metal oxide film 407) preferably has a thickness of larger than 10 nm and smaller than or equal to 100 nm.

Note that after the metal oxide film 427 is formed, the protective insulating film 419 is further formed over the metal oxide film 427, so that the transistor 500 in FIG. 4H can be formed. The protective insulating film 419 can be formed using a material and a method which are similar to those of the insulating film 409 described later. Further, the protective insulating film 419 can also be patterned when the metal oxide film 427 is patterned into the metal oxide film 407 in a later step. The protective insulating film 419 may be patterned in a different step from that of the metal oxide film 427.

Next, a resist mask is formed over the metal oxide film 427 by a third photolithography step, the metal oxide film 407 functioning as a channel protective film is formed by etching, and then, the resist mask is removed (see FIG. 5C). The resist masks for forming the metal oxide film 407 may be formed by an ink-jet method. Formation of the resist mask by an inkjet method needs no photomask; thus, manufacturing cost can be reduced. For the etching of the metal oxide film 427, wet etching, dry etching, or both of them may be employed.

Light exposure at the time when the resist mask is formed in the third photolithography step may be performed using ultraviolet light, KrF laser light, or ArF laser light. The channel length L of the transistor formed later is determined, depending on the width in the channel length direction of the metal oxide film 407 functioning as a channel protective film. When light exposure is performed for a channel length L of smaller than 25 nm, the light exposure when the resist mask is formed in the third photolithography step may be performed using extreme ultraviolet light having an extremely short wavelength of several nanometers to several tens of nanometers, for example. In the light exposure by extreme ultraviolet light, the resolution is high and the focus depth is large. Thus, the channel length L of the transistor formed later can be reduced, whereby the operation speed of a circuit can be increased.

Here, patterning is performed so that the width in the channel length direction of the metal oxide film 407 is smaller than that of the oxide semiconductor film 403, whereby the source electrode 405 a and the drain electrode 405 b are in contact with part of a top surface of the oxide semiconductor film 403. Thus, the width in the channel length direction of the metal oxide film 407 is made small, so that the channel length of the transistor is made small; therefore, an increase in operation speed and a reduction in power consumption of the transistor can be achieved.

In an etching step of the metal oxide film 427, when etching selectivity ratio of the metal oxide film 427 to the oxide semiconductor film 403 cannot be high, part of a region of the oxide semiconductor film 403 which does not overlap with the metal oxide film 427 might be removed. In this case, the thickness of the region of the oxide semiconductor film 403 which does not overlap with the metal oxide film 427 is reduced.

In the case where the oxide semiconductor film 403 is not processed into an island shape in the above step, the oxide semiconductor film 403 may be processed into an island shape simultaneously with the metal oxide film 427. By thus patterning the oxide semiconductor film 403 and the metal oxide film 427 simultaneously, the number of photolithography steps can be reduced. Further, the oxide semiconductor film 403 and the metal oxide film 427 are patterned using the same mask; thus, side edges in a channel length direction of the oxide semiconductor film 403 are aligned with those of the metal oxide film 407. In this case, the gate insulating film 402, the oxide semiconductor film 403, and the metal oxide film 427 are preferably formed successively without being exposed to the air.

The metal oxide film 427 is not necessarily processed into an island shape. For example, openings in which parts of the oxide semiconductor film 403 are exposed may be provided so that the oxide semiconductor film 403 can be electrically connected to the source electrode 405 a and the drain electrode 405 b in a later step. With such a structure, the transistor 320 in FIG. 3B, the transistor 340 in FIG. 4B, the transistor 360 in FIG. 4D, and the transistor 380 in FIG. 4F can be formed.

Next, a conductive film for forming the source electrode and the drain electrode (including a wiring formed in the same layer as the source electrode and the drain electrode) is formed to cover the metal oxide film 407 and the oxide semiconductor film 403. As the conductive film used for the source electrode layer and the drain electrode layer, for example, a metal film including an element selected from Al, Cr, Cu, Ta, Ti, Mo, and W, a metal nitride film including any of the above elements as its component (e.g., a titanium nitride film, a molybdenum nitride film, or a tungsten nitride film), or the like can be used. A high-melting-point metal film of Ti, Mo, W, or the like or a metal nitride film of any of these elements (a titanium nitride film, a molybdenum nitride film, or a tungsten nitride film) may be stacked on one of or both a bottom side and a top side of a metal film of Al, Cu, or the like. A metal film having a high melting point of Ti, Mo, W, or the like or a metal nitride film of any of these elements (a titanium nitride film, a molybdenum nitride film, or a tungsten nitride film) may be stacked on one of or both a lower side and an upper side of a metal film of Al, Cu, or the like. Alternatively, the conductive film to be the source electrode and the drain electrode may be formed using a conductive metal oxide. As conductive metal oxide, indium oxide (In₂O₃), tin oxide (SnO₂), zinc oxide (ZnO), indium oxide-tin oxide alloy (In₂O₃—SnO₂; abbreviated to ITO), indium oxide-zinc oxide alloy (In₂O₃—ZnO), or any of these metal oxide materials in which silicon or silicon oxide is contained can be used.

Then, a resist mask is formed over the conductive film in a fourth photolithography step, and selective etching is performed so that the source electrode 405 a and the drain electrode 405 b are formed, and then, the resist mask is removed (see FIG. 5D). When ultraviolet light, KrF laser light, or ArF laser light is used for light exposure for forming the resist mask in a third photolithography step, a distance between a lower end of the source electrode 405 a and a lower end of the drain electrode 405 b that are adjacent to each other over the metal oxide film 407; thus, ultraviolet light, KrF laser light, or ArF laser light is preferably used in a fourth photolithography step.

In order to reduce the number of photomasks and steps in the photolithography step, an etching step may be performed with the use of a resist mask formed using a multi-tone mask which is a light-exposure mask through which light is transmitted so as to have various intensities. A resist mask formed with the use of a multi-tone mask has various thicknesses and further can be changed in shape by etching; therefore, the resist mask can be used in a plurality of etching steps for different patterns. Therefore, a resist mask corresponding to at least two or more kinds of different patterns can be formed by one multi-tone mask. Thus, the number of light-exposure masks can be reduced and the number of corresponding photolithography steps can also be reduced, whereby simplification of a manufacturing process can be realized.

The metal oxide film 407 is provided in a region overlapping with a channel formation region of the oxide semiconductor film 403; thus, damage to the channel formation region by etching of the conductive film (e.g., damage due to plasma or etchant in etching) can be prevented. Thus, a semiconductor device including an oxide semiconductor with stable electric characteristics can be provided.

Next, the insulating film 409 covering the source electrode 405 a and the drain electrode 405 b is preferably formed over and in contact with the metal oxide film 407 (see FIG. 5E). It is desirable to use a material with which a charge trapping center can be formed at the interface with the metal oxide film 407 when the material is in contact with the metal oxide film 407, for the insulating film 409. By using such a material for the insulating film 409, charge is trapped at the interface between the insulating film 409 and the metal oxide film 407, so that it is possible to sufficiently suppress trapping of charge at the interface between the metal oxide film 407 and the oxide semiconductor film 403.

The insulating film 409 may be formed using an inorganic film, for example, a single layer or a stack of any of oxide insulating films such as a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, and an aluminum oxynitride film, and nitride insulating films such as a silicon nitride film, a silicon nitride oxide film, an aluminum nitride film, and an aluminum nitride oxide film. For example, a silicon oxide film and a silicon nitride film are sequentially formed to be stacked from the metal oxide film 407 side by a sputtering method. The insulating film 409 preferably contains a constituent different from the constituents contained in the oxide semiconductor film 403 and the metal oxide film 407. Note that the insulating film 409 is preferably a silicon oxide film in order that impurities such as hydrogen and moisture may be removed from the metal oxide film 407 efficiently in a step of heat treatment performed on the oxide semiconductor film 403 later. In addition, the energy gap of the insulating film 409 is desirably larger than that of the metal oxide film 407 because the insulating film 409 is in contact with the metal oxide film 407.

Note that it is not necessary to limit the material of the insulating film 409 to the above as long as a charge trapping center can be formed at the interface between the insulating film 409 and the metal oxide film 407. Further, treatment through which a charge trapping center is formed may be performed on the interface between the insulating film 409 and the metal oxide film 407. As such treatment, plasma treatment and treatment for adding an element (ion implantation or the like) are given, for example

After that, second heat treatment is preferably performed while part of the oxide semiconductor film 403 (channel formation region) is in contact with the metal oxide film 407. The second heat treatment is performed at a temperature higher than or equal to 250° C. and lower than or equal to 700° C., preferably higher than or equal to 450° C. and lower than or equal to 600° C. The temperature of the first heat treatment is preferably lower than the strain point of the substrate.

The second heat treatment may be performed in an atmosphere of nitrogen, oxygen, ultra-dry air (air in which a water content is lower than or equal to 20 ppm, preferably lower than or equal to 1 ppm, further preferably lower than or equal to 10 ppb), or a rare gas (argon, helium, or the like). Note that it is preferable that water, hydrogen, or the like be not contained in the atmosphere of nitrogen, oxygen, ultra-dry air, or a rare gas. It is also preferable that the purity of nitrogen, oxygen, or the rare gas which is introduced into a heat treatment apparatus be set to higher than or equal to 6N (99.9999%), preferably higher than or equal to 7N (99.99999%) (that is, the impurity concentration is lower than or equal to 1 ppm, preferably lower than or equal to 0.1 ppm).

The second heat treatment is performed while the oxide semiconductor film 403 and the metal oxide film 407 are in contact with each other. Thus, oxygen which is one of main constituent materials of the oxide semiconductor and may be reduced due to the dehydration (or dehydrogenation) treatment can be supplied from the metal oxide film 407 containing oxygen to the oxide semiconductor film 403. Accordingly, charge trapping centers in the oxide semiconductor film 403 can be decreased. Through the above steps, the oxide semiconductor film 403 can be highly purified to be electrically i-type (intrinsic). In addition, impurities are removed from the metal oxide film 407 at the same time by this heat treatment, and the metal oxide film 407 can be highly purified.

Note that in this embodiment, the second heat treatment is performed after formation of the insulating film 409; however, there is no particular limitation on the timing of the second heat treatment as long as it is performed after formation of the metal oxide film 407. For example, the second heat treatment may be performed after the metal oxide film 407 is formed. Alternatively, when the insulating film 409 is formed by stacking, for example, a silicon oxide film and a silicon nitride film, the second heat treatment may be performed after the silicon oxide film is formed over the metal oxide film 407 and then, the silicon nitride film may be formed thereover. Furthermore, the first heat treatment and the second heat treatment may be successively performed, the first heat treatment may also serve as the second heat treatment, or the second heat treatment may also serve as the first heat treatment.

By performing at least one of the first heat treatment and the second heat treatment as described above, the oxide semiconductor film 403 can be purified so as not to contain impurities other than main components as little as possible. The number of carriers in the highly-purified oxide semiconductor film 403 is very small (close to zero), and the carrier concentration is less than 1×10¹⁴/cm³, preferably less than 1×10¹²/cm³, further preferably less than 1×10¹¹/cm³.

Through the above steps, the transistor 310 is formed (see FIG. 5E). The transistor 310 is a transistor including the oxide semiconductor film 403 which is highly purified and from which impurities such as hydrogen, moisture, a hydroxyl group, or hydride (also referred to as a hydrogen compound) are removed. Therefore, variation in the electric characteristics of the transistor 310 is suppressed and the transistor 310 is electrically stable.

In the transistor including the metal oxide film 407, generation of a parasitic channel on the back channel side of the oxide semiconductor film 403 can be prevented. By preventing the generation of a parasitic channel on the back channel side of the oxide semiconductor film 403 in the transistor 310, variation in the threshold voltage can be suppressed, whereby the reliability of the transistor can be improved.

Note that after the insulating film 409 is formed, the conductive film 410 is further formed in a region over the insulating film 409 and overlapping with a channel formation region of the oxide semiconductor film 403, so that the transistor 330 in FIG. 4A and the transistor 340 in FIG. 4B can be formed. Note that if the gate electrode 401 and the gate insulating film 402 are not provided, transistors 350 and 360 in FIGS. 4C and 4D can be formed. The conductive film 410 can be formed using a material and a method which are similar to those of the gate electrode 401. The conductive film 410 is provided so as to overlap with the channel formation region of the oxide semiconductor film 403, whereby in a bias-temperature stress test (referred to as a BT test) for examining the reliability of the transistor 340, the amount of change in the threshold voltage of the transistor 340 before and after the BT test can be further reduced. Note that the conductive film 410 may have the same potential as the gate electrode 401 or have a potential different from that of the gate electrode 401 and may function as a second gate electrode. The potential of the conductive film 410 may be GND or 0 V, or the conductive film 410 may be in a floating state.

Although not illustrated, a protective insulating film may be further formed so as to cover the transistor 310. As the protective insulating film, a silicon nitride film, a silicon nitride oxide film, an aluminum nitride film, or the like can be used.

A planarizing insulating film may be formed over the transistor 310. The planarization insulating film can be formed of a heat-resistant organic material such as acrylic, polyimide, benzocyclobutene, polyamide, or epoxy. Other than such organic materials, it is also possible to use a low-dielectric constant material (a low-k material), a siloxane-based resin, PSG (phosphosilicate glass), BPSG (borophosphosilicate glass), or the like. Note that the planarizing insulating film may be formed by stacking a plurality of insulating films formed of any of these materials.

As described above, in the transistor according to this embodiment, the top surface portion of the oxide semiconductor film is provided with the metal oxide film containing a constituent similar to that of the oxide semiconductor film. Thus, the metal oxide film containing a material compatible with the oxide semiconductor film is provided in contact with the oxide semiconductor film, whereby suppressed is trapping of charge or the like, which is generated due to the operation of a semiconductor device, at the interface between the oxide semiconductor film and the metal oxide film. Consequently, the oxide semiconductor film can be less adversely affected by charge, which suppresses fluctuation in the threshold voltage of the transistor due to trapping of charge at the interface of the oxide semiconductor film.

Further, an insulating film containing a different constituent from the metal oxide film and the oxide semiconductor film is formed in contact with a surface of the metal oxide film, which is opposite to the surface in contact with the oxide semiconductor film. Thus, an insulator containing a material with which a charge trapping center can be formed at the interface is provided in contact with the metal oxide film, whereby the charge can be trapped preferentially at the interface between the metal oxide film and the insulator compared to the interface between the oxide semiconductor film and the metal oxide film. Consequently, the oxide semiconductor film can be less adversely affected by charge, which further suppresses fluctuation in the threshold voltage of the transistor due to trapping of charge at the interface of the oxide semiconductor film.

The oxide semiconductor film used for the active layer of the transistor is an oxide semiconductor film highly purified to be electrically i-type (intrinsic) by heat treatment in which impurities such as hydrogen, moisture, a hydroxyl group, or a hydride (also referred to as a hydrogen compound) are removed from the oxide semiconductor and oxygen which is a major constituent of the oxide semiconductor and is reduced concurrently with a step of removing impurities is supplied. The transistor including the oxide semiconductor film highly purified in such a manner has electric characteristics which are less likely to change, and thus is electrically stable.

When charge is trapped at the interface of the oxide semiconductor film, the threshold voltage of the transistor shifts (for example, when positive charge is trapped on the back channel side, the threshold voltage of the transistor shifts in a negative direction). As one of factors of such charge trapping, the model in which cations (or atoms which are sources of the cations) travel and are trapped can be supposed, for example. In the transistor including an oxide semiconductor, such cation sources may be hydrogen atoms. In the disclosed invention, the highly purified oxide semiconductor is used and is in contact with the stack of the metal oxide film and the insulating film, so that it is possible to suppress even charge trapping caused by hydrogen, which may be caused in the above model. The above model is supposed to be realized when the ionization rate of hydrogen is, for example, about 10%

As described above, a semiconductor device including an oxide semiconductor having stable electric characteristics can be provided. Therefore, a semiconductor device with high reliability can be provided.

The structures, methods, and the like described in this embodiment can be combined as appropriate with any of the structures, methods, and the like described in the other embodiments.

Embodiment 2

A semiconductor device (also referred to as a display device) with a display function can be manufactured using the transistor an example of which is described in Embodiment 1. Some or all of driver circuits including the transistors can be formed over a substrate where a pixel portion is formed, whereby a system-on-panel can be obtained.

In FIG. 6A, a sealant 4005 is provided to surround a pixel portion 4002 provided over a first substrate 4001, and the pixel portion 4002 is sealed with the sealant 4005 and the second substrate 4006. In FIG. 6A, a scan line driver circuit 4004 and a signal line driver circuit 4003 each are formed using a single crystal semiconductor film or a polycrystalline semiconductor film over a substrate prepared separately, and mounted in a region different from the region surrounded by the sealant 4005 over the first substrate 4001. Various signals and potentials are supplied to the signal line driver circuit 4003 and the scan line driver circuit 4004 each of which is separately formed, and the pixel portion 4002, from flexible printed circuits (FPCs) 4018 a and 4018 b.

In FIGS. 6B and 6C, the sealant 4005 is provided to surround the pixel portion 4002 and the scan line driver circuit 4004 which are provided over the first substrate 4001. The second substrate 4006 is provided over the pixel portion 4002 and the scan line driver circuit 4004. Thus, the pixel portion 4002 and the scan line driver circuit 4004 are sealed together with a display element, by the first substrate 4001, the sealant 4005, and the second substrate 4006. In FIGS. 6B and 6C, the signal line driver circuit 4003 is formed using a single crystal semiconductor film or a polycrystalline semiconductor film over a substrate prepared separately, and mounted in a region different from the region surrounded by the sealant 4005 over the first substrate 4001. In FIGS. 6B and 6C, various signals and potentials are supplied to the separately formed signal line driver circuit 4003, the scan line driver circuit 4004, and the pixel portion 4002, from an FPC 4018.

Although FIGS. 6B and 6C each show the example in which the signal line driver circuit 4003 is formed separately and mounted on the first substrate 4001, the present invention is not limited to this structure. The scan line driver circuit may be separately formed and then mounted, or only part of the signal line driver circuit or part of the scan line driver circuit may be separately formed and then mounted.

Note that a method for connecting a separately formed driver circuit is not particularly limited, and a chip on glass (COG) method, a wire bonding method, a tape automated bonding (TAB) method, or the like can be used. FIG. 6A shows an example in which the signal line driver circuit 4003 and the scan line driver circuit 4004 are mounted by a COG method. FIG. 6B shows an example in which the signal line driver circuit 4003 is mounted by a COG method. FIG. 6C shows an example in which the signal line driver circuit 4003 is mounted by a TAB method.

The display device includes in its category a panel in which a display element is sealed, and a module in which an IC such as a controller is mounted on the panel.

Note that a display device in this specification means an image display device, a display device, or a light source (including a lighting device). The display device also includes the following modules in its category: a module to which a connector such as an FPC, a TAB tape, or a TCP is attached; a module having a TAB tape or a TCP at the tip of which a printed wiring board is provided; and a module in which an integrated circuit (IC) is directly mounted on a display element by a COG method.

The pixel portion and the scan line driver circuit provided over the first substrate include a plurality of transistors and any of the transistors which are described in Embodiment 1 can be applied.

As the display element provided in the display device, a liquid crystal element (also referred to as a liquid crystal display element) or a light-emitting element (also referred to as a light-emitting display element) can be used. The light-emitting element includes, in its category, an element whose luminance is controlled by a current or voltage, and specifically includes, in its category, an inorganic electroluminescent (EL) element, an organic EL element, and the like. Furthermore, a display medium whose contrast is changed by an electric effect, such as electronic ink, can be used.

One embodiment of the semiconductor device will be described with reference to FIG. 7, FIG. 8, and FIG. 9. FIG. 7, FIG. 8, and FIG. 9 correspond to cross-sectional views taken along line M-N in FIG. 6B.

As shown in FIG. 7, FIG. 8, and FIG. 9, the semiconductor device includes a connection terminal electrode 4015 and a terminal electrode 4016. The connection terminal electrode 4015 and the terminal electrode 4016 are electrically connected to a terminal included in the FPC 4018 through an anisotropic conductive film 4019.

The connection terminal electrode 4015 is formed of the same conductive film as a first electrode layer 4030. The terminal electrode 4016 is formed of the same conductive film as a source electrode and a drain electrode of transistors 4010 and 4011.

Each of the pixel portion 4002 and the scan line driver circuit 4004 provided over the first substrate 4001 includes a plurality of transistors. In FIG. 7, FIG. 8, and FIG. 9, the transistor 4010 included in the pixel portion 4002 and the transistor 4011 included in the scan line driver circuit 4004 are shown as an example.

In this embodiment, any of the transistors shown in Embodiment 1 can be applied to the transistors 4010 and 4011. Variation in the electric characteristics of the transistors 4010 and 4011 is suppressed and the transistors 4010 and 4011 are electrically stable. As described above, a semiconductor device with high reliability as the semiconductor devices shown in FIG. 7, FIG. 8, and FIG. 9 can be obtained.

The transistor 4010 provided in the pixel portion 4002 is electrically connected to the display element to constitute a display panel. A variety of display elements can be used as the display element as long as display can be performed.

An example of a liquid crystal display device using a liquid crystal element as a display element is shown in FIG. 7. In FIG. 7, a liquid crystal element 4013 is a display element including the first electrode layer 4030, a second electrode layer 4031, and a liquid crystal layer 4008. Note that the insulating films 4032 and 4033 serving as alignment films are provided so that the liquid crystal layer 4008 is interposed therebetween. The second electrode layer 4031 is formed on the second substrate 4006 side. The first electrode layer 4030 and the second electrode layer 4031 are stacked with the liquid crystal layer 4008 interposed therebetween.

A columnar spacer 4035 is obtained by selective etching of an insulating film and is provided in order to control the thickness (a cell gap) of the liquid crystal layer 4008. Alternatively, a spherical spacer may be used.

In the case where a liquid crystal element is used as the display element, a thermotropic liquid crystal, a low-molecular liquid crystal, a high-molecular liquid crystal, a polymer dispersed liquid crystal, a ferroelectric liquid crystal, an anti-ferroelectric liquid crystal, or the like can be used. Such a liquid crystal material exhibits a cholesteric phase, a smectic phase, a cubic phase, a chiral nematic phase, an isotropic phase, or the like depending on conditions.

Alternatively, liquid crystal exhibiting a blue phase for which an alignment film is unnecessary may be used. A blue phase is one of liquid crystal phases generated just before a cholesteric phase changes into an isotropic phase while temperature of cholesteric liquid crystal is increased. Since the blue phase appears only in a narrow temperature range, a liquid crystal composition in which several weight percent or more of a chiral material is mixed is used for the liquid crystal layer in order to improve the temperature range. The liquid crystal composition which includes a liquid crystal showing a blue phase and a chiral agent has a short response time of 1 msec or less, has optical isotropy, which makes the alignment process unneeded, and has a small viewing angle dependence. In addition, since an alignment film does not need to be provided and rubbing treatment is unnecessary, electrostatic discharge damage caused by the rubbing treatment can be prevented and defects and damage of the liquid crystal display device can be reduced in the manufacturing process. Thus, productivity of the liquid crystal display device can be increased.

The specific resistivity of the liquid crystal material is 1×10⁹ Ω·cm or more, preferably 1×10¹¹ Ω·cm or more, further preferably 1×10¹² Ω·cm or more. Note that the specific resistivity in this specification is measured at 20° C.

The size of a storage capacitor provided in the liquid crystal display device is set considering the leakage current of the transistor provided in the pixel portion or the like so that charge can be held for a predetermined period. Since the transistor including a high-purity oxide semiconductor film is used, a storage capacitor having capacitance which is ⅓ or less, preferably ⅕ or less with respect to a liquid crystal capacitance of each pixel is sufficient to be provided.

In the transistor used in this embodiment, which uses a highly-purified oxide semiconductor film, the current in an off state (the off-state current) can be made small. Therefore, an electrical signal such as an image signal can be held for a long period, and a writing interval can be set long when the power is on. Consequently, frequency of refresh operation can be reduced, which leads to an effect of suppressing power consumption.

The field-effect mobility of the transistor including a highly-purified oxide semiconductor film used in this embodiment can be relatively high, whereby high-speed operation is possible. Thus, by using the transistor in a pixel portion of the liquid crystal display device, a high-quality image can be provided. In addition, since the transistors can be separately provided in a driver circuit portion and a pixel portion over one substrate, the number of components of the liquid crystal display device can be reduced.

For the liquid crystal display device, a twisted nematic (TN) mode, an in-plane-switching (IPS) mode, a fringe field switching (FFS) mode, an axially symmetric aligned micro-cell (ASM) mode, an optical compensated birefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, an antiferroelectric liquid crystal (AFLC) mode, and the like can be used.

A normally black liquid crystal display device such as a transmissive liquid crystal display device utilizing a vertical alignment (VA) mode is preferable. The vertical alignment mode is one of methods of controlling alignment of liquid crystal molecules of a liquid crystal display panel. The vertical alignment mode is a mode in which liquid crystal molecules are aligned vertically to a panel surface when voltage is not applied. Some examples are given as the vertical alignment mode. For example, a multi-domain vertical alignment (MVA) mode, a patterned vertical alignment (PVA) mode, an Advanced Super View (ASV) mode, and the like can be used. Moreover, it is possible to use a method called domain multiplication or multi-domain design, in which a pixel is divided into some regions (subpixels) and molecules are aligned in different directions in their respective regions.

In the display device, a black matrix (a light-blocking layer), an optical member (an optical substrate) such as a polarizing member, a retardation member, or an anti-reflection member, and the like are provided as appropriate. For example, circular polarization may be employed by using a polarizing substrate and a retardation substrate. In addition, a backlight, a side light, or the like may be used as a light source.

In addition, with the use of a plurality of light-emitting diodes (LEDs) as a backlight, a time-division display method (a field-sequential driving method) can be employed. With the field-sequential driving method, color display can be performed without using a color filter.

As a display method in the pixel portion, a progressive method, an interlace method, or the like can be employed. Color elements controlled in a pixel at the time of color display are not limited to three colors: R, G, and B (R, G, and B correspond to red, green, and blue respectively). For example, R, G, B, and W (W corresponds to white), or R, G, B, and one or more of yellow, cyan, magenta, and the like can be used. The sizes of display regions may be different between respective dots of color elements. Note that the present invention is not limited to the application to a display device for color display but can also be applied to a display device for monochrome display.

Alternatively, as the display element included in the display device, a light-emitting element utilizing electroluminescence can be used. Light-emitting elements utilizing electroluminescence are classified according to whether a light-emitting material is an organic compound or an inorganic compound. In general, the former is referred to as an organic EL element, and the latter is referred to as an inorganic EL element.

In an organic EL element, by application of voltage to a light-emitting element, electrons and holes are separately injected from a pair of electrodes into a layer containing a light-emitting organic compound, and current flows. The carriers (electrons and holes) are recombined, and thus the light-emitting organic compound is excited. The light-emitting organic compound returns to a ground state from the excited state, thereby emitting light. Owing to such a mechanism, such a light-emitting element is referred to as a current-excitation light-emitting element.

The inorganic EL elements are classified according to their element structures into a dispersion-type inorganic EL element and a thin-film inorganic EL element. A dispersion-type inorganic EL element has a light-emitting layer where particles of a light-emitting material are dispersed in a binder, and its light emission mechanism is donor-acceptor recombination type light emission that utilizes a donor level and an acceptor level. A thin-film inorganic EL element has a structure where a light-emitting layer is sandwiched between dielectric layers, which are further sandwiched between electrodes, and its light emission mechanism is localized type light emission that utilizes inner-shell electron transition of metal ions. Note that an example of an organic EL element as a light-emitting element is described here.

In order to extract light emitted from the light-emitting element, it is acceptable as long as at least one of a pair of electrodes is transparent. Then a transistor and a light-emitting element are formed over a substrate. The light-emitting element can have any of the following structure: a top emission structure in which light is extracted through the surface opposite to the substrate; a bottom emission structure in which light is extracted through the surface on the substrate side; or a dual emission structure in which light is extracted through the surface opposite to the substrate and the surface on the substrate side.

An example of a light-emitting device using a light-emitting element as a display element is shown in FIG. 8. A light-emitting element 4513 which is a display element is electrically connected to the transistor 4010 provided in the pixel portion 4002. The light-emitting element 4513 has a stacked-layer structure of the first electrode layer 4030, an electroluminescent layer 4511, and the second electrode layer 4031 but is not limited to this structure. The structure of the light-emitting element 4513 can be changed as appropriate depending on a direction in which light is extracted from the light-emitting element 4513, or the like.

A partition wall 4510 can be formed using an organic insulating material or an inorganic insulating material. It is particularly preferable that the partition wall 4510 be formed using a photosensitive resin material to have an opening portion over the first electrode layer 4030 so that a sidewall of the opening portion is formed as a tilted surface with continuous curvature.

The electroluminescent layer 4511 may be formed with either a single layer or a stacked layer of a plurality of layers.

A protective film may be formed over the second electrode layer 4031 and the partition wall 4510 in order to prevent entry of oxygen, hydrogen, moisture, carbon dioxide, or the like into the light-emitting element 4513. As the protective film, a silicon nitride film, a silicon nitride oxide film, a diamond like carbon (DLC) film, or the like can be formed. In a space sealed with the first substrate 4001, the second substrate 4006, and the sealant 4005, a filler 4514 is provided and tightly sealed. It is preferable that the light-emitting element be packaged (sealed) with a cover material with high air-tightness and little degasification or a protective film (such as a laminate film or an ultraviolet curable resin film) so that the light-emitting element is not exposed to the outside air, in this manner.

As the filler 4514, an ultraviolet curable resin or a thermosetting resin can be used as well as an inert gas such as nitrogen or argon, and polyvinyl chloride (PVC), acrylic, polyimide, an epoxy resin, a silicone resin, polyvinyl butyral (PVB), ethylene vinyl acetate (EVA), or the like can be used. For example, nitrogen is used for the filler.

If needed, an optical film, such as a polarizing plate, a circularly polarizing plate (including an elliptically polarizing plate), a retardation plate (a quarter-wave plate or a half-wave plate), or a color filter, may be provided as appropriate on a light-emitting surface of the light-emitting element. Further, the polarizing plate or the circularly polarizing plate may be provided with an anti-reflection film. For example, anti-glare treatment by which reflected light can be diffused by projections and depressions on the surface so as to reduce the glare can be performed.

An electronic paper in which electronic ink is driven can be provided as the display device. The electronic paper is also called an electrophoretic display device (electrophoretic display) and has advantages in that it has the same level of readability as regular paper, it has less power consumption than other display devices, and it can be set to have a thin and light form.

An electrophoretic display device can have various modes. An electrophoretic display device contains a plurality of microcapsules dispersed in a solvent or a solute, each microcapsule containing first particles which are positively charged and second particles which are negatively charged. By applying an electric field to the microcapsules, the particles in the microcapsules move in opposite directions to each other and only the color of the particles gathering on one side is displayed. Note that the first particles and the second particles each contain pigment and do not move without an electric field. Moreover, the first particles and the second particles have different colors (which may be colorless).

Thus, an electrophoretic display device is a display device that utilizes a so-called dielectrophoretic effect by which a substance having a high dielectric constant moves to a high-electric field region.

A solution in which the above microcapsules are dispersed in a solvent is referred to as electronic ink. This electronic ink can be printed on a surface of glass, plastic, cloth, paper, or the like. Furthermore, by using a color filter or particles that have a pigment, color display can also be achieved.

Note that the first particles and the second particles in the microcapsules may each be formed of a single material selected from a conductive material, an insulating material, a semiconductor material, a magnetic material, a liquid crystal material, a ferroelectric material, an electroluminescent material, an electrochromic material, and a magnetophoretic material, or formed of a composite material of any of these.

As an electronic paper, a display device using a twisting ball display method can be used. The twisting ball display method refers to a method in which spherical particles each colored in white and black are arranged between a first electrode layer and a second electrode layer which are electrode layers used for a display element, and a potential difference is generated between the first electrode layer and the second electrode layer to control orientation of the spherical particles, so that display is performed.

FIG. 9 shows an active matrix electronic paper as one embodiment of a semiconductor device. The electronic paper in FIG. 9 is an example of a display device using a twisting ball display method.

Between the first electrode layer 4030 connected to the transistor 4010 and the second electrode layer 4031 provided on the second substrate 4006, spherical particles 4613 each of which includes a black region 4615 a, a white region 4615 b, and a cavity 4612 around the regions which is filled with liquid, are provided. A space around the spherical particles 4613 is filled with a filler 4614 such as a resin. The second electrode layer 4031 corresponds to a common electrode (counter electrode). The second electrode layer 4031 is electrically connected to a common potential line.

Note that in FIG. 7, FIG. 8, and FIG. 9, a flexible substrate as well as a glass substrate can be used as the first substrate 4001 and the second substrate 4006. For example, a plastic substrate having light-transmitting properties can be used. For plastic, a fiberglass-reinforced plastics (FRP) plate, a polyvinyl fluoride (PVF) film, a polyester film, or an acrylic resin film can be used. A sheet with a structure in which an aluminum foil is sandwiched between PVF films or polyester films can also be used.

The insulating layer 4021 can be formed using an organic insulating material or an inorganic insulating material. Note that an organic insulating material having heat resistance, such as an acrylic resin, a polyimide, a benzocyclobutene-based resin, a polyamide, or an epoxy resin is preferably used as a planarizing insulating film. Other than such organic insulating materials, it is possible to use a low-dielectric constant material (a low-k material), a siloxane-based resin, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), or the like. The insulating layer may be formed by stacking a plurality of insulating films formed of these materials.

There is no particular limitation on the method for forming the insulating layer 4021, and the insulating layer 4021 can be formed, depending on a material thereof, by a sputtering method, a spin coating method, a dipping method, a spray coating method, a droplet discharging method (e.g., an ink jet method, a screen printing method, or an offset printing method), a roll coating method, a curtain coating method, a knife coating method, or the like.

The display device performs display by transmitting light from a light source or a display element. Thus, the substrates and the thin films such as insulating films and conductive films provided in the pixel portion where light is transmitted have light-transmitting properties with respect to light in the visible-light wavelength range.

The first electrode layer and the second electrode layer (each of which may be called a pixel electrode layer, a common electrode layer, a counter electrode layer, or the like) for applying voltage to the display element may have light-transmitting properties or light-reflecting properties, which depends on the direction in which light is extracted, the position where the electrode layer is provided, and the pattern structure of the electrode layer.

A light-transmitting conductive material such as indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium tin oxide (hereinafter referred to as ITO), indium zinc oxide, or indium tin oxide to which silicon oxide is added, can be used for the first electrode layer 4030 and the second electrode layer 4031.

The first electrode layer 4030 and the second electrode layer 4031 can be formed using one kind or plural kinds selected from metal such as tungsten (W), molybdenum (Mo), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), cobalt (Co), nickel (Ni), titanium (Ti), platinum (Pt), aluminum (Al), copper (Cu), or silver (Ag); an alloy thereof; and a nitride thereof.

A conductive composition containing a conductive high molecule (also referred to as a conductive polymer) can be used for the first electrode layer 4030 and the second electrode layer 4031. As the conductive high molecule, a so-called π-electron conjugated conductive polymer can be used. For example, polyaniline or a derivative thereof, polypyrrole or a derivative thereof, polythiophene or a derivative thereof, and a copolymer of two or more of aniline, pyrrole, and thiophene or a derivative thereof can be given.

Since the transistor is easily broken due to static electricity or the like, a protective circuit for protecting the driver circuit is preferably provided. The protective circuit is preferably formed using a nonlinear element.

As described above, by using any of the transistors shown in Embodiment 1, a semiconductor device having a high reliability can be provided. Note that the transistors described in Embodiment 1 can be applied to not only semiconductor devices having the display functions described above but also semiconductor devices having a variety of functions, such as a power device which is mounted on a power supply circuit, a semiconductor integrated circuit such as an LSI, and a semiconductor device having an image sensor function of reading information of an object.

This embodiment can be implemented in appropriate combination with the structures described in the other embodiments.

Embodiment 3

A semiconductor device disclosed in this specification can be applied to a variety of electronic appliances (including game machines). Examples of electronic appliances are a television set (also referred to as a television or a television receiver), a monitor of a computer or the like, a camera such as a digital camera or a digital video camera, a digital photo frame, a mobile phone handset (also referred to as a mobile phone or a mobile phone device), a portable game machine, a portable information terminal, an audio reproducing device, a large-sized game machine such as a pachinko machine, and the like. Examples of electronic appliances each including the semiconductor device described in the above embodiment will be described.

FIG. 10A illustrates a laptop personal computer, which includes a main body 3001, a housing 3002, a display portion 3003, a keyboard 3004, and the like. By applying the semiconductor device described in Embodiment 1 or 2, the laptop personal computer can have high reliability.

FIG. 10B is a portable information terminal (PDA) which includes a display portion 3023, an external interface 3025, an operation button 3024, and the like in a main body 3021. A stylus 3022 is included as an accessory for operation. By applying the semiconductor device described in Embodiment 1 or 2, the portable information terminal (PDA) can have higher reliability.

FIG. 10C illustrates an example of an electronic book reader. For example, an electronic book reader 2700 includes two housings, a housing 2701 and a housing 2703. The housing 2701 and the housing 2703 are combined with a hinge 2711 so that the electronic book reader 2700 can be opened and closed with the hinge 2711 as an axis. With such a structure, the electronic book reader 2700 can operate like a paper book.

A display portion 2705 and a display portion 2707 are incorporated in the housing 2701 and the housing 2703, respectively. The display portion 2705 and the display portion 2707 may display one image or different images. When the display portion 2705 and the display portion 2707 display different images, for example, text can be displayed on a display portion on the right side (the display portion 2705 in FIG. 10C) and graphics can be displayed on a display portion on the left side (the display portion 2707 in FIG. 10C). By applying the semiconductor device described in Embodiment 1 or 2, the electronic book reader 2700 can have high reliability.

FIG. 10C illustrates an example in which the housing 2701 is provided with an operation portion and the like. For example, the housing 2701 is provided with a power switch 2721, operation keys 2723, a speaker 2725, and the like. With the operation key 2723, pages can be turned. Note that a keyboard, a pointing device, or the like may also be provided on the surface of the housing, on which the display portion is provided. Furthermore, an external connection terminal (an earphone terminal, a USB terminal, or the like), a recording medium insertion portion, and the like may be provided on the back surface or the side surface of the housing. Moreover, the electronic book reader 2700 may have a function of an electronic dictionary.

The electronic book reader 2700 may have a configuration capable of wirelessly transmitting and receiving data. Through wireless communication, desired book data or the like can be purchased and downloaded from an electronic book server.

FIG. 10D illustrates a mobile phone, which includes two housings, a housing 2800 and a housing 2801. The housing 2801 includes a display panel 2802, a speaker 2803, a microphone 2804, a pointing device 2806, a camera lens 2807, an external connection terminal 2808, and the like. In addition, the housing 2800 includes a solar cell 2810 having a function of charge of the mobile phone, an external memory slot 2811, and the like. Further, an antenna is incorporated in the housing 2801. By applying the semiconductor device described in Embodiment 1 or 2, the mobile phone can have high reliability.

Further, the display panel 2802 is provided with a touch panel. A plurality of operation keys 2805 which are displayed as images is illustrated by dashed lines in FIG. 10D. Note that a boosting circuit by which a voltage output from the solar cell 2810 is increased to be sufficiently high for each circuit is also included.

In the display panel 2802, the display orientation can be appropriately changed depending on a usage pattern. Further, the display device is provided with the camera lens 2807 on the same surface as the display panel 2802, and thus it can be used as a video phone. The speaker 2803 and the microphone 2804 can be used for videophone calls, recording and playing sound, and the like as well as voice calls. Moreover, the housings 2800 and 2801 in a state where they are opened as illustrated in FIG. 10D can be slid so that one overlaps the other; therefore, the size of the mobile phone can be reduced, which makes the mobile phone suitable for being carried.

The external connection terminal 2808 can be connected to an AC adapter and various types of cables such as a USB cable, and charging and data communication with a personal computer are possible. Moreover, a larger amount of data can be saved and moved by inserting a recording medium to the external memory slot 2811.

Further, in addition to the above functions, an infrared communication function, a television reception function, or the like may be provided.

FIG. 10E illustrates a digital video camera which includes a main body 3051, a display portion A 3057, an eyepiece 3053, an operation switch 3054, a display portion B 3055, a battery 3056, and the like. By applying the semiconductor device described in Embodiment 1 or 2, the digital video camera can have high reliability.

FIG. 10F illustrates an example of a television set. In a television set 9600, a display portion 9603 is incorporated in a housing 9601. The display portion 9603 can display images. Here, the housing 9601 is supported by a stand 9605. By applying the semiconductor device described in Embodiment 1 or 2, the television set can have high reliability.

The television set 9600 can be operated by an operation switch of the housing 9601 or a separate remote controller. Further, the remote controller may be provided with a display portion for displaying data output from the remote controller.

Note that the television set 9600 is provided with a receiver, a modem, and the like. With the use of the receiver, general television broadcasting can be received. Furthermore, when the display device is connected to a communication network with or without wires via the modem, one-way (from a sender to a receiver) or two-way (between a sender and a receiver or between receivers) information communication can be performed.

This embodiment can be implemented in appropriate combination with the structures described in the other embodiments.

This application is based on Japanese Patent Application serial No. 2010-090539 filed with the Japan Patent Office on Apr. 9, 2010, the entire contents of which are hereby incorporated by reference. 

The invention claimed is:
 1. A semiconductor device comprising: a first transistor comprising: a first conductive layer over an insulating surface; a first insulating layer over the first conductive layer; a first oxide semiconductor layer over the first conductive layer; an oxide layer over the first oxide semiconductor layer; a second conductive layer over the oxide layer; a third conductive layer over the oxide layer; and a fourth conductive layer over the oxide layer, wherein the second conductive layer is in contact with the first oxide semiconductor layer through a first opening provided in the oxide layer, wherein the third conductive layer is in contact with the first oxide semiconductor layer through a second opening provided in the oxide layer, wherein the fourth conductive layer overlaps with the first oxide semiconductor layer with a second insulating layer interposed therebetween, and wherein the fourth conductive layer is configured to function as a gate of the first transistor, a second transistor having a top-gate structure, the second transistor comprising: the first insulating layer over the insulating surface; a second oxide semiconductor layer over the first insulating layer; the oxide layer over the second oxide semiconductor layer; a fifth conductive layer over the oxide layer; a sixth conductive layer over the oxide layer; and a seventh conductive layer over the oxide layer, wherein the fifth conductive layer is in contact with the second oxide semiconductor layer through a third opening provided in the oxide layer, wherein the sixth conductive layer is in contact with the second oxide semiconductor layer through a fourth opening provided in the oxide layer, wherein the seventh conductive layer overlaps with the second oxide semiconductor layer with the second insulating layer interposed therebetween, and wherein the seventh conductive layer is configured to function as a gate of the second transistor.
 2. A semiconductor device according to claim 1, wherein the first transistor is electrically connected to the second transistor.
 3. A semiconductor device according to claim 1, wherein each of the first oxide semiconductor layer and the second oxide semiconductor layer comprises In, Ga, and Zn. 